There have been proposed a variety of photovoltaic elements such as solar cells and power sources for commercial and home appliances. They utilize pn junctions formed by ion implantation or thermal diffusion of impurities into a single crystal substrate of silicon (Si) or gallium arsenide (GaAs), or by epitaxial growth of an impurity-doped layer on such single crystal substrate. However, there is a disadvantage for any of these photovoltaic elements that they are still costly because of using an expensive specific single crystal substrate. Hence, they have not yet come into general use as solar cells or power sources in commercial and home appliances used by the general public. In order to solve this problem, there have been proposed a photovoltaic element in which there is utilized a pin junction formed by amorphous silicon (hereinafter referred to as "a-Si") semiconductor films laminated on an inexpensive substrate of a non-single crystal material such as glass, metal, ceramic, synthetic resin, etc. by way of a glow discharge decomposition method. This photovoltaic element does not provide a photoelectric conversion efficiency as high as that provided by the foregoing pn junction photovoltaic element in which a single crystal substrate is used. However, this photovoltaic element can be relatively easily produced and is of low production cost, and because of this, it is used as a power source in some kinds of appliances with very small power consumption such as electronic calculators and wrist watches.
In this pin junction amorphous silicon photovoltaic element, the Fermi level of the a-Si semiconductor having a good photoconductive property lies a little toward the conduction band from the center of the band gap and the electric field strength at the interface of the p-i junction is greater than that at the interface of the n-i junction. In this respect, it is advantageous to impinge light from the side of the p-type semiconductor layer in order to provide a desirable photoelectric conversion efficiency.
For the p-type semiconductor layer, it is desired to be formed of such a semiconductor film that does not absorb light and does not have defects since the light to be absorbed within the p-type semiconductor layer does not contribute to generation of photoelectric current in the case where defects acting as recombination centers are present therein. In view of this, for the semiconductor film to constitute the p-type semiconductor layer in the pin junction a-Si photovoltaic element, studies have been made on amorphous silicon carbide films (hereafter referred to as "a-SiC film") which are of wide band gap and also on microcrystal line silicon films (hereinafter referred to as ".mu.C-Si film") which are known as indirect semiconductor films having small absorption coefficients and which are considered to hardly absorb light when they are of 100 to 200.ANG. in thickness even in the case where they are of narrow band gap. As for the a-SiC semiconductor film, there is an advantage that its band gap can be widened by increasing the composition ratio of the constituent carbon atoms. However, there is a disadvantage that when its band gap is more than 2.1 eV, its film quality is markedly worsened. Therefore, there is a limit for the a-SiC semiconductor film to be used as the p-type semiconductor layer in a pin heterojunction photovoltaic element.
As for the uC-Si semiconductor film, there is still a disadvantage that its band gap is narrow in any case and the quantity of light absorbed thereby is remarkable. Particularly, when the incident light is such that it contains short-wavelength light in a large proportion, the quantity of light absorbed becomes great.
In view of this, in order to provide a desirable pin heterojunction photovoltaic element of the type wherein light is impinged from the side of the p-type semiconductor layer, it necessitates the use of a p-type semiconductor film having a desirably wide band gap and a minimized defect density as the p-type semiconductor layer.
The same situation is present also in the case of a pin heterojunction photovoltaic element of the type wherein light is impinged from the side of the n-type semiconductor layer. That is, the n-type semiconductor layer is required to be constituted by such an n-type semiconductor film having a desirably wide band gap and a minimized defect density.
Further, in the case of a so-called tandem stacked type photovoltaic element or a triple cell tandem stacked type photovoltaic element comprising a plurality of stacked cells, each cell of which comprises a pin heterojunction photovoltaic element in which the residual components of light which are left not absorbed by the upper cell are absorbed by the lower cell to obtain a sufficient photoelectric conversion, both the p-type semiconductor layer and the n-type semiconductor layer of each of the cells are required to have a desirably wide band gap and a minimized defect density.
Further, for any of the foregoing photovoltaic elements, it is required for the material to constitute the p-type or n-type semiconductor layer to be such that it can be directly deposited on a non-single crystal substrate of glass, metal, ceramic or synthetic resin in a desired state and does not give any negative effect to the i-type semiconductor layer laminated thereon.
As semiconductor films capable of providing a wide band which satisfy the foregoing requirements, GaP semiconductor films have been proposed by Japanese Patent Laid-open No. 116673/1981 (called "literature 1" hereinafter), Japanese Patent Laid-open No. 6874/1986 (called "literature 2" hereinafter), Japanese Patent Laid-open No. 189629/1986 (called "literature 3" hereinafter) and Japanese Patent Laid-open No. 189630/1986 (called "literature 4" hereinafter).
That is, literature 1 mentions a pin heterojunction solar cell in which either the p-type or n-type semiconductor layer is comprised of a p-type or n-type amorphous GaP semiconductor film (that is, a-GaP semiconductor film) prepared by the glow discharge decomposition method and the i-type semiconductor layer is comprised of an a-Si semiconductor film containing fluorine atoms (F). Literature 1 does not mention anything about a crystalline GaP semiconductor film (that is a poly-GaP semiconductor film) which is apparently distinguished from said a-GaP semiconductor film. In addition, literature 1 does not describe anything about the characteristics required for a solar cell using the said pin heterojunction photovoltaic element.
Literature 2 discloses pin junction solar cells in which the p-type semiconductor layer is comprised of a GaP film and each of the i-type and n-type semiconductor layers is comprised of a-Si film and it says that the pin junction solar cells are remarkably improved in the short-circuit current density (Isc), open-circuit voltage (Voc) and fill-factor (FF) in comparison with the known pin junction solar cells. But literature 2 does not mention anything about their solar cell characteristics and particular values of the parameters. Literature 2 does not mention anything about the process of forming the GaP film or the evaluation thereof. Literature 2 gives details neither about the a-Si film nor about the a-SiGe film.
Literatures 3 and 4 are concerned with methods of forming semiconductor films containing group III-V elements of the Periodic Table by way of the HR-CVD method (Hydrogen Radical Assisted CVD method). But neither of literatures 3 and 4 mentions anything about GaP semiconductor films.
Further, none of literatures 1 to 4 mentions a tandem type photovoltaic element or a triple cell tandem stacked type photovoltaic element.
Against this background, there is an increased social demand to provide an inexpensive photovoltaic element which exhibits a high photoelectric conversion efficiency particularly for short wavelength light and which is practically usable as a solar cell and also as a power source in various appliances.